In the traditional production of polystyrene (PS) foams using an extrusion process, it was common to utilize as blowing agents one or more halocarbons, such as methyl chloride, ethyl chloride, chlorocarbons, fluorocarbons (including HFCs) and chlorofluorocarbons (CFCs) including dichlorodifluoromethane, fluorohydrocarbons or chlorofluorohydrocarbons (which are also referred to as “soft CFCs”, “HCFCs” or “HFCs”). Examples of such halocarbons include a range of CFCs such as CFC-11 (chlorotrifluoromethane), CFC-12 (dichlorodifluoromethane), and CFC-113 (1,2,2-trifluoro-1,1,2-tri-chloroethane), and hydrohalocarbons, also referred to as “soft” CFCs, HCFCs and HFCs, including HCFC-22 (chlorodifluoromethane), HCFC-123 (1,1-dichloro-2,2,2-trifluoroethane), HCFC-142b (1-chloro-1,1-difluoroethane), HFC-134a (1,1,1,2-tetrafluoroethane), HFC-152a (1,1-difluoroethane), and HCFC-141b (1,1-dichloro-1-fluoroethane.
The general procedure utilized in the preparation of extruded synthetic foam bodies generally includes the steps of melting a base polymeric composition, incorporating one or more blowing agents and other additives into the polymeric melt under conditions that provide for the thorough mixing of the blowing agent and the polymer while preventing the mixture from foaming prematurely, e.g., under pressure. This mixture is then typically extruded through a single or multi-stage extrusion die to cool and reduce the pressure on the mixture, allowing the mixture to foam and produce a foamed product. As will be appreciated, the relative quantities of the polymer(s), blowing agent(s) and additives, the temperature and the manner in which the pressure is reduced will tend to affect the qualities and properties of the resulting foam product. As will also be appreciated, the foamable mixture is maintained under a relatively high pressure until it passes through an extrusion die and is allowed to expand in a region of reduced pressure. Although “reduced” relative to the pressure at the extrusion die, the reduced pressure region may actually be maintained at a pressure above atmospheric pressure, for example up to about 2 atm or even more in some applications, may be maintained at a pressure below atmospheric pressure, for example down to about 0.25 atm or even less in some applications. Further, unless indicated otherwise, all references to pressure provided herein are stated as the absolute pressure.
The solubility of chlorofluorocarbons and certain alkanes in polystyrene tends to reduce the extrusion melt viscosity and improve cooling of expanded polystyrene (PS) melts. For example, the combination of pentane and a chlorofluorocarbon such as Freon 11 and 12 is partially soluble in polystyrene and has been used for generating polystyrene foams that exhibited a generally acceptable appearance and physical properties such as surface finish, cell size and distribution, orientation, shrinkage and stiffness.
However, in response to the apparent contribution of such CFC compounds to the reduction of the ozone layer in Earth's stratosphere, the widespread use and accompanying atmospheric release of such compounds in applications such as aerosol propellants, refrigerants, foam-blowing agents and specialty solvents has recently been drastically reduced or eliminated by government regulation. Although certain of the “soft” CFCs such as certain hydrofluorocarbons (HFC's) including 1,1,1,2-tetrafluoroethane (HFC-134a) and 1,1-difluoroethane (HFC-152a) are thought to be much more ozone friendly and have been considered as alternative blowing agents. However, these alternative compounds are expensive, tend to be less soluble in polystyrene, tend to have higher thermal conductivity than HCFC's and may still contribute to global warming.
Hydrocarbons such as pentane, hexane, cyclopentane and other homologs of this series have also been considered, but they are highly flammable and volatile, thereby raising both safety and VOC emission concerns. Carbon dioxide is an attractive candidate as a blowing agent, from both the environmental and economic standpoints. The challenges associated with successfully using CO2 as a blowing agent are, however, significant in light of the relatively low solubility, high diffusivity and poor processability of CO2 in polystyrene resins. CO2 also has an increased thermal conductivity relative to that of HCFC-142b and HFC-134a, with CO2-blown foam exhibiting about 17% and about 10% lower overall product insulation values respectively than corresponding foams produced with HCFC-142b and HFC-134a.
Other previous attempts have utilized alcohols, such as ethanol, or hydrocarbon, such as cyclopentane, in conjunction with CO2 to improve the processability and enable the production of extruded polystyrene foam board having desired or target physical and thermal properties. The problems with co-blowing agents such as alcohols or hydrocarbons are their flammability, safety and the negative impact on flame performance and insulation properties of the end product.
Conventional processes include polymer foam processes for making thermoformed articles wherein the blowing agent comprises a mixture of at least an atmospheric gas and at least one volatile plasticizing blowing agent. Previous attempts to mix normally liquid hydrocarbons and normally gaseous blowing agents have generally achieved only limited success and have tended to require great care in order to produce acceptable polymer foams using highly-volatile blowing agents such as carbon dioxide.